Level monitoring device for determining and monitoring a fill level of a medium in the process area of a vessel

ABSTRACT

A fill level measuring device for ascertaining and monitoring fill level of a medium in the process space of a container by means of a microwave travel time measuring method. The device includes: measurement transmitter; and an antenna unit, which is constructed at least of a hollow conductor and a radiating element, wherein a process isolation element is inserted into the hollow conductor for process isolation between measurement transmitter and the process contacting, radiating element. The process isolation element is made of a ceramic material and includes at least one glass layer, via which the process isolation element is directly glass bonded in the hollow conductor in a glass bonding region.

The present invention relates to a fill level measuring device forascertaining and monitoring fill level of a medium in the process spaceof a container, as such device is defined in the preamble of claim 1.

One measuring method, out of a number of measuring methods forascertaining fill level in a container, is the travel time measuringmethod. With this measuring method, for example, microwaves are radiatedvia an antenna device and the waves reflected on the surface of themedium are detected, with the travel time of the measuring signal beinga measure of distance. From half the travel time, the fill level of themedium in a container can, in this way, be ascertained. The echo curverepresents, here, the entire curve of the signal as a function of time,with each measured value of the echo curve corresponding to an amplitudeof an echo signal reflected at a surface at a certain distance. Thetravel time measuring method is essentially divided into two evaluationmethods: In the time difference measurement method, the time, which abroadband wave, signal pulse requires for a traveled distance, isdetermined. In the frequency modulated, continuous wave method(FMCW—Frequency Modulated Continuous Wave), the transmitted,frequency-modulated, high frequency signal is compared with thereflected, received, frequency-modulated, high frequency signal. In thefollowing, no restriction is made to any particular method ofmeasurement.

In the case of certain process applications, fill level measuringdevices are exposed to extreme conditions, for example hightemperatures, high pressures and/or chemically aggressive substances. Inparticular, microwave, fill level measuring devices contain temperature,and/or pressure, sensitive components. These include, for example,measuring device electronics and transmission and/or reception elementsfor the microwaves.

Insertion of a hermetically sealed, process isolation element into thehollow conductor of the antenna ensures highest possible safety, since asecond “safety element” seals the process, during an isolating of themodular, measurement active parts, such as e.g. a couplingelement/exciter element or the measuring device electronics, from themeasurement passive parts, such as e.g. the antenna, for maintenance orrepair.

This problematic and a solution therefor are considered in EP 0 943 902A1. There, a fill level measuring device working with microwaves isdescribed for high temperature applications. The device has an antennaand includes a process isolation element in the hollow conductor regionof the antenna. A glass window, among others, is described as a processisolation element. These glass windows protect the sensitive componentsof the fill level measuring devices against extreme measurementconditions, such as high temperatures, high pressures, and chemicallyaggressive media. A disadvantage of this design of the process isolationelement is that the glass window must, because of the availableproduction technology, for example due to the different materialexpansions, be provided in a thin-walled metal sleeve. This sleeve withthe glass window must be soldered or welded in further, complicated,working steps into the hollow conductor. This requires a high additionalwork effort associated with the production of the antenna of the filllevel measuring device. In addition, with the many working steps,manufacturing costs and safety risk are increased due to manufacturingerrors.

US 2005/0253751 A1 describes a modular construction of a horn antenna.The process isolation element is constructed in the form of a ceramic,matching cone that is introduced into the hollow conductor and sealed bygraphite packing rings. This design has the disadvantage that a sealingagainst gas diffusion and a temperature resistant, process isolation arenot achieved.

In DE 199 50 429 A1, a ceramic process isolation element is describedthat is shrunk fit into the hollow conductor. Disadvantageous, here, isthat, despite polished bounding surfaces on the process isolationelement and in the waveguide, no seal is achieved. Further, the largecompressive forces that act on the ceramic, process isolation elementcan lead to stress cracks.

A disadvantage of the aforementioned examples of embodiments of a stateof the art process isolation element is that manufacture is very complexand expensive. In order to obtain a connection impervious to gasdiffusion between a ceramic and a surrounding metal, hollow conductor,only a soldering procedure is well-known according to the state of theart. In such case, the ceramic, as process isolation element, is firstmetallized on the surface in complex working steps, then soldered into asoldering sleeve, which has a coefficient of thermal expansion similarto that of the ceramic (e.g. Kovar), and this finally is welded into astainless steel, hollow conductor. Other joining techniques, such as,for example, shrink fitting at high temperature, always have a certainleakage rate and are not impervious to gas diffusion, as alreadymentioned.

An object of the invention is to provide a fill level measuring devicehaving a gas diffusion resistant, process isolation element for processisolation, which does not exhibit the disadvantages specified above, andwhich, in particular, can be produced economically and simply.

This object of the invention is achieved by the features set forth inclaim 1.

Advantageous further developments of the invention are specified in thedependent claims.

Further details, characteristics, and advantages of the subject matterof the invention will be understood from the following description incombination with the associated drawings, in which advantageousembodiments of the invention are presented. In the embodiments of theinvention presented in the figures of the drawing, in order not toclutter and for simplification, components or groups of components,which correspond in their structure and/or in their function, are givenequal reference characters. The figures of the drawing show as follows:

FIG. 1 a schematic representation of an antenna unit equipped, filllevel measuring device of process measurements technology, and

FIG. 2 longitudinal, sectional view of the hollow conductor of theantenna unit of FIG. 1, having a process isolation element according tothe invention.

FIG. 1 shows a fill level measuring device 1 of process measurementstechnology used for determining fill level 2 in a container 4. The filllevel measuring device is composed, fundamentally, of an antenna unit 7and a measurement transmitter 23. The antenna unit 7 includes in thehollow conductor 8, in this example of an embodiment, a processisolation element 11 of the invention. The fill level measuring device1, which is mounted via a process connection 35 onto a container 4,ascertains, for example by the travel time measurement method, the level2 of a medium 3 and/or fill substance in the container 4. The antennaunit 7 is, in this example of an embodiment, provided in the form of ahorn antenna. The process isolation element 11 of the invention is alsodeployable with other types of antenna units, such as, for example, rodantennas, planar antennas, parabolic antennas, and in measuring systemsof time domain reflectometry working with a waveguide-led microwave. Theantenna unit 7 can be divided into two fundamental, functional units—thehollow conductor 8 and the radiating element 12.

Provided in the measurement transmitter 23 is a transmitting/receivingunit 27, in which the microwave measuring signals 6 are produced. Via acoupling element 33, the microwave measuring signals 6 are coupled intothe hollow conductor 8 of the antenna unit 7. The coupling element 33 isinstalled in the hollow conductor 8 via a gas diffusion blocking, glassfeedthrough. The microwave measurement signals 6 coupled into the hollowconductor 8 of the antenna unit 7 are radiated, in given cases, througha filling element 36, from the radiation element 10, as sent, ortransmission, signals S into the process space 5 with a predeterminedradiation characteristic. Usually the aim is to have a radiationcharacteristic of the microwave measuring signals exhibiting a planarwave front, in order to avoid travel time differences in the reflectionsignals R. The microwave measuring signals 6 transmitted into themeasurement space 5 are reflected on the surface of the medium 3 andreceived, after a certain travel time, back at thetransmitting/receiving unit 27. From the travel time of the microwavemeasurement signal 6, the fill level 2 of the medium 3 in the container4 is determined.

The control/evaluation unit 26 in the measurement transmitter 23 has thetask of evaluating the received reflection signals R of the microwavemeasuring signals 6, using further processing of the measurement signal6 by signal processing and special, signal evaluating algorithms, as anecho curve, and therefrom, the travel time, or the fill level 2, isascertained.

The control/evaluation unit 26 communicates via a communicationinterface 28 with a remote control location and/or with additionalfill-level measuring devices 1, which are not explicitly shown. Via thesupply line 29, the fill-level measuring device 1 can be supplied withthe required energy. This additional supply line 29 for energy supply ofthe fill-level measuring device 1 is absent, when the device is a socalled two-conductor measuring device, whose communication and energysupply take place via the fieldbus 30 exclusively and simultaneously viaa two-wire line. The data transmission, or communication, via thefieldbus 30 occurs, for example, according to the CAN, HART, PROFIBUSDP, PROFIBUS FMS, PROFIBUS PA, or FOUNDATION FIELDBUS standard.

FIG. 2 presents a sectional view of an example of an embodiment of thehollow conductor 8 with the glass bonded, ceramic, process isolatingelement 11 of the invention. According to the invention, a ceramicmatching cone 22 transmissive for microwaves is provided, which is meltbonded in a metal, hollow conductor 8 by means of a 1-2 mm thick,annular glass layer 15. The hollow conductor 8 is, in this case,embodied in the form of a round, hollow conductor. However, any otherform of hollow conductor 8 can be used for the installation of theinvention of the matching cone 22 by means of glass bonding. Throughglass bonding of the ceramic matching cone 22 in the hollow conductor 8,there is achieved a gas diffusion blocking, microwave transmissive,process isolation, which is well suited for use in the face of hightemperatures, high pressures and aggressive process conditions.Disadvantageous with a glass layer 15 contacting the process, however,is that glass is corroded by steam. In order to avoid this corrosion ofthe thin glass layer 15, a graphite packing ring 16 is placed in frontof the glass layer 15 for protecting it. The sealing action of thegraphite packing ring 16 is achieved by executing the hollow conductor 8in two parts and, by a screwed connecting of the two elements 9,10 ofthe hollow conductor 8, a compressive force is exerted on the graphitepacking ring 16. Additionally, a corrosion resistant coating 37 can beapplied partially on the glass layer 15. This corrosion resistantcoating 37 can be produced, for example, through vapor deposition of achromium/gold coating. A graphite packing ring 16 as corrosionprotection of the glass layer 15 is, due to the corrosion protectionfrom the application of a corrosion resistant coating 37, then no longerabsolutely necessary. However, the graphite packing ring 16 thenprovides an additional sealing action.

Through introduction of the process isolating element 11 into the hollowconductor 8, the wave resistance of the conductor system is altered. Inorder to match this wave resistance, the hollow conductor is tapered,especially in the matching region 14. The process isolating element 11includes an matching cone 22 having a cylindrical shape, which tapers inthe matching region 14 toward both end faces at a certain angle 24, andwhich has, thus, on both sides at least one step or multistep, conicalappendages. The embodiment of the process isolating element 11 asmatching cone 22 has, as a result, that the maximum diameter of the coneis larger than the minimum diameter of the hollow conductor 8 at theposition of maximum necking. For this reason, it can be necessary tomake the hollow conductor 8 in two parts at the location of the glassbonding, or introduction, and to provide there a location of separation20.

In this example of an embodiment, the hollow conductor 8 is, such asalready mentioned, constructed of two units, a first element 9 and asecond element 10, which are connected with one another via a screwedconnection 19. At the location of separation 21, the first element 9 andthe second element 10 are welded together gas tightly via a radiallysurrounding, weld seam on the outer surface 32, or at the location ofseparation 20. This two part construction of the hollow conductor 8 isnecessary in this example of an embodiment, since, first of all, theprocess isolating element 11 is embodied as matching cone 22 formatching the wave resistance, and, secondly, because, for protection ofthe glass layer against steam, an additional graphite packing ring isplaced in front of it as a supplemental sealing element.

For lessening the attenuation of the microwaves 6, for example, a hollowspace 18 is provided in the process isolating element 11 and filled witha dielectric, fill material 38. This fill material 38 has, relative tothe ceramic of the matching cone 22, a much smaller permittivity, ordielectric constant, whereby the intensity of the microwaves 6 is notstrongly attenuated by the fill material 38. Furthermore, selected asfill material 38 is, for example, a material having a small thermalexpansion, e.g. ROHACELL, a material comprising hollow glass spheres oradditional, temperature compensated fillers.

The matching cone 22 is, according to the invention, inserted in thefirst element 9 of the hollow conductor 8. In such case, a glasssubstrate is introduced either as powder or prefabricated ring into afree gap in the glass bonding region 13 and melted by a predeterminedtemperature cycle in a furnace. Used as glass substrate are, forexample, glasses usual for glass feedthroughs. In the melted state, theglass layer 15 brings about with the metal, hollow conductor 8 and/orthe ceramic matching cone 22 a material bonded interlocking, gasdiffusion blocking connection. Furthermore, another option is to apply aglass layer 15 directly on the ceramic body of the matching cone 22 andto use this prefabricated part in the seat provided therefor in thefirst element 9 of the hollow conductor 8. The application of a thinglass layer 15 of some millimeters can occur, for example, also using achemical or physical gas phase deposit coating method (CVD, PVD). Theheating of the glass layer 15 can, for example, also be achieved byradiating highly energetic microwaves with a high intensity focused onthe glass layer 15, so that a strong heating is produced only zonally inthe glass bonding region 13. Once the matching cone 22 is bonded via theglass layer 15 in the first element 9 of the hollow conductor 8 and, ingiven cases, a corrosion resistant coating 37 applied, then the graphitepacking ring 16 is pressed via the screwed connection 19 of the secondelement 10 of the hollow conductor 8 fixedly into the cavity providedbelow the glass layer 15. Advantageously, the expansion coefficients ofthe materials of the matching cone 22, the hollow conductor 8 and theglass layer 15 are so matched to one another, that no extreme stresses,or even stress cracks, occur in the material composite. The matchingcone 22 is, for example, made of a technical-grade, aluminum oxideceramic.

For increasing the quality of sealing and the corrosion resistance, theceramic matching cone 22 bonded by the glass in the hollow conductor 8,and the inner surfaces 31 of the hollow conductor 8 can even be providedwith an additional coating 17. This coating can be produced, forexample, by a simple chemical or physical gas phase deposit, coatingmethod (CVD, PVD).

A further advantage of the glass bonding in comparison to soldering isthat no complicated surface preparation, such as polishing, orhardening, or curing, of the ceramic and no expensive materials, such ase.g. Kovar for the soldering sleeve, are required. Moreover, themanufacture of the process isolating element 11 and its glass bonding inthe hollow conductor 8 are clearly easier and therewith significantlymore cost effective.

The process isolating element 11 of the invention delivers otheradvantages, for instance, that the coupling element 33 in the case ofcondensate formation, and/or the electronics and the coupling element33, can be removed, since in a first safety stage, themeasurement-inactive parts of the antenna unit 7, such as, for example,the flange-plating of the filling element 36 seal the process to theoutside and the process isolating element 11 forms a second safety stage(second line of defense). In this way, an option is provided, in thecase of an alteration or repair of the fill-level measuring device 1, tomount the measurement transmitter 23 on the antenna unit 7, with theprocess being in a sealed state. Depending on embodiment andapplication, the fill-level measuring device 1 can be composed ofdifferent modules. An alteration of the fill-level measuring device 1 touse another type of coupling, e.g. step, or pin, coupling, or anotherfrequency, e.g. 6 GHz or 26 GHz, is possible through the isolation ofthe active parts from the passive parts with the process being in asealed state. The coupling element 33 is, for example, modularlyembodied and can be inserted via a screwed connection into the hollowconductor 8.

LIST OF REFERENCE CHARACTERS

TABLE 1  1 fill level measuring device  2 fill level  3 medium  4container  5 process space  6 microwaves, microwave measuring signal  7antenna unit  8 hollow conductor  9 first element 10 second element 11process isolating element 12 radiating element 13 glass bonding region14 matching region 15 glass layer 16 graphite packing ring 17 coating 18hollow space 19 screwed connection 20 weld seam 21 separation 22matching cone 23 measurement transmitter 24 angle 25 stage 26 controlunit (open or closed loop) 27 transmitting/receiving unit 28communication interface 29 supply line 30 communication line 31 innersurface 32 outer surface 33 coupling element 34 glass feedthrough 35process connection 36 fill body 37 corrosion resistant coating 38 fillsubstance R reflection signals S transmission signals

1-14. (canceled)
 15. A fill-level measuring device for ascertaining andmonitoring fill level of a medium located in a process space of acontainer by means of a method measuring travel time of microwaves, saiddevice comprising: a measurement transmitter; an antenna unit, which isconstructed at least of a hollow conductor and a radiating element; anda process isolating element transmissive for microwaves is inserted insaid hollow conductor, between said measurement transmitter and saidradiating element contacting the process, for process isolation,wherein: said process isolating element comprises a ceramic material andat least one glass layer, via which said process isolating element isbonded in a glass bonding region directly to said hollow conductor. 16.The apparatus as claimed in claim 15, further comprising: at least onegraphite packing ring on the process side, which has an additionalsealing action and protects said at least one glass layer of the directglass bonding on the process-side against corrosion from the medium. 17.The apparatus as claimed in claim 15, wherein: at least one partiallyapplied, corrosion resistant coating is provided on the process-side onsaid glass layer, which protects said glass layer of the direct glassbonding on the process-side against corrosion from the medium.
 18. Theapparatus as claimed in claim 16, wherein: said hollow conductor isconstructed of a plurality of parts, including at least a first elementand a second element.
 19. The apparatus as claimed in claim 18, wherein:securement by means of a screwed connection of said first element andsaid second element is provided, which effects an additional sealingaction due to compression exerted on said graphite packing ring.
 20. Theapparatus as claimed in claim 15, wherein: a surrounding weld seam isprovided on the outer surface of said hollow conductor, which secures alocation of separation of said first element and said second element ofsaid hollow conductor against rotation.
 21. The apparatus as claimed inclaim 15, wherein: said at least one glass layer has a thickness of 0.5to 5 millimeters.
 22. The apparatus as claimed in claim 15, wherein: onthe process-side, a microwave transmissive, corrosion resistant coatingis provided on said glass-bonded, process isolating element and/or theinner surface of said hollow conductor.
 23. The apparatus as claimed inclaim 15, wherein: said ceramic, process isolating element is embodiedas an matching cone, whose cross section in a matching region conicallytapers starting from a glass bonding region of the zonal, direct glassbonding of the matching cone in said hollow conductor, in at least onestep and at at least one angle.
 24. The apparatus as claimed in claim15, wherein: a hermetically sealed, hollow space is provided in theinterior of said ceramic, process isolating element.
 25. The apparatusas claimed in claim 24, wherein: a microwave transmissive fill materialwith a smaller permittivity is provided in the hermetically sealed,hollow space of said ceramic, process isolating element.
 26. Theapparatus as claimed in claim 15, wherein: a single step or multistep,linearly decreasing inner diameter of said hollow conductor is providedin a matching region of said process isolating element in the directionof the zonal, direct glass bonding.
 27. The apparatus as claimed inclaim 15, wherein: provided as material for said hollow conductor is astainless steel tube or a ceramic, or plastic, tube coated with metal oninner surfaces.
 28. The apparatus as claimed in claim 15, wherein: aplanar antenna, a parabolic antenna, a horn antenna or a rod antenna isprovided as said antenna unit.